When molten plastic is processed by an injection molding machine, the plastic enters a mold cavity where it is cooled to form a desired part shape. As the cooling occurs, the plastic contracts within the cavity. As a result of this contraction, the part actually shrinks in size, and sink marks or low spots often occur on the surface of the part. Shrink and sink marks have caused major problems for injection molders since injection molding was first developed. Several methods have been developed in an attempt to eliminate these problems. Some examples include gas-assisted injection molding, structural foam molding, liquid gas assisted molding, etc. In addition, foaming agents have been used in the molding process for mixing with molten plastic in order to generate inert gases in the plastic. These gases provide internal pressure in the plastic which enables the plastic to more fully fill the cavity of the mold and packs the plastic against the cavity walls. This, in turn, helps reduce sink on the surface of the plastic parts. Also, gas counterpressure in the mold cavity has been used to improve surface smoothness of molded parts.
These prior art methods are all problematic due to the large number of variables in the molding process. Varying injection pressures and injection speeds, varying melt pressures and temperatures, varying cavity conditions, and uncontrolled venting of gases all contribute to an unstable molding environment. These various problems in the molding process create burning and scission of polymer chains and create internal stresses within the plastic which remain in the plastic as the plastic material cools in the cavity. These internal stresses cause shrink, sink, and warpage of the plastic part to be molded. In addition, these various molding problems lead to degradation of the plastic material as it is processed through an injection molding machine. In general, erratic variations in pressure, temperature, and injection speed create material breakdown and cause internal problems in the plastic which show up in the final molded product.
Another disadvantage of prior art systems is that the plastic melt flow in these systems experiences changes in pressure due to changes in cavity geometry as the molten plastic moves into the cavity of the mold. These pressure changes cause certain areas of the cavity to be filled more quickly than other areas resulting in different cooling characteristics in different areas of the cavity. These cooling variations cause inconsistencies in the direction of plastic solidification, which results in surface stresses, weld lines, or sink.
Gas assisted injection molding is a process for forming a hollow part in which a pressurized assist gas is injected into the molten material either in the nozzle or within the mold cavity. The assist gas forms a hollow bubble within the part which reduces the part weight, thereby reducing material costs. A major problem with this process is that it is very difficult to control the movement of the bubble within the molten plastic in the cavity. Therefore, the hollow part often has walls of uneven thickness.
U.S. Pat. No. 5,558,824 attempted to solve the problem of bubble movement within the mold cavity by prepressurizing the mold cavity with an inert gas, and controlling the release of the gas from the cavity to prevent blow out of the bubble from the interior of the molten plastic. It provides pressure sensors for sensing the pressure of the gas in the mold cavity and the pressure of the gas injected into the bubble.
The primary problem with the disclosure provided in the '824 patent is that there is no means provided for controlling the pressures acting on the bubble, and therefore bubble formation and movement cannot be controlled. Using this process, Mr. Shah will be unable to control the bubble because it is too difficult to respond in real time by altering the pressures in the cavity and within the bubble by means of valves outside the mold cavity and pressurized gas sources. Furthermore, gas is too compressible to provide the capability of controlling the melt pressure of the molten material in the cavity during injection in real time. Using this system, one cannot recognize mold cavity resistance or resistance from partial solidification, and therefore cannot respond accordingly and control the growth of the bubble. For instance, if the molten material is injected through a very thin area in the cavity, the mold cavity resistance increases, and an increase in bubble pressure will be sensed, but the source of this bubble pressure will not be known, and no means are provided for responding, other than by altering the discharge rate of gas from the cavity, which will be ineffective.
FIGS. 15a-c illustrate the flow of a typical gas assisted melt front through a mold cavity in accordance with the prior art. The melt M includes an assist gas bubble B therein. As the melt front reaches the cavity flow restriction R, as illustrated at FIG. 15b, this flow restriction will cause an increase in the pressure of the melt, which will pressurize the bubble, and will often cause the bubble to blow through the forward surface of the melt front, which would result in a scrapped part. If the melt front were to flow past the restriction, the bubble B will likely move closely adjacent the corner of the restriction R, thus resulting in a very thin section T to be formed in the final part as molded. This thin section greatly reduces the structural integrity of the final part as molded. Also, the material M may become thin at the forward edge of the melt front, and the bubble may blow through this thin portion at any time.
It is desirable to use a balanced injection molding process in which the pressure of the molten plastic is continuously controlled as the plastic moves through the injection molding machine. It is further desirable for an injection molding process to balance pressures acting upon the molten material in order to eliminate the above referenced problems caused by variations in polymer chain conditions so as to reduce internal stresses in the plastic. The ultimate goal of such an injection molding process is to produce a final product which nearly perfectly matches the cavity surface of the mold, is fully relieved of internal stresses which lead to shrink, sink, and warpage, and has greatly improved mechanical properties. In addition, part weight may be reduced, which will provide significant material savings to the manufacturers of such products.